Slower to crack we hear (5)
I believe the answer is:
brake
'slower' is the definition.
(I've seen this before)
'crack we hear' is the wordplay.
'crack' becomes 'break'.
'we hear' indicates a 'sounds like' (homophone) clue.
'break' sounds like 'BRAKE'.
'to' acts as a link.
(Other definitions for brake that I've seen before include "Constraint" , "Device for slowing or stopping car" , "Decelerate" , "Thicket; fern; slow down" , "Device for slowing vehicle" .)